A fuel cell is an electricity generating system converting the chemical reaction energy of a chemical reaction between oxygen and hydrogen into electric energy. The hydrogen used in the reaction may be contained in hydrocarbon materials such as methanol, ethanol, and natural gas.
Recently developed polymer electrolyte membrane fuel cells (PEMFC) have excellent output characteristic, low operating temperatures, and fast starting and response characteristics. So, the PEMFC has a wide range of applications and may be used in a mobile power source for vehicles, in a distributed power source for buildings, or in a small power source for electronic apparatus.
A fuel cell system employing the PEMFC includes a main body, also referred to as a stack, a reformer for reforming a fuel to generate hydrogen and for supplying the hydrogen to the stack, and an air pump or a fan for supplying oxygen to the stack.
The stack generates electric energy at a predetermined output level through an electrochemical reaction between the hydrogen, supplied from the reformer, and the oxygen, supplied by the air pump or fan, and delivers the electric energy to a predetermined load.
The fuel cell system includes an auxiliary power supply device, such as a rechargeable secondary battery, for supplying an auxiliary power to the fuel pump and for starting up the fuel cell system. For example, at the time of starting up the fuel cell system, the fuel pump and some other parts are driven with the power supplied from the auxiliary power supply device.
In addition, in the conventional fuel cell system, in a case where electric energy required by the load exceeds the output electric energy of the stack and the fuel cell is in an electrically overloaded state, the auxiliary power supply device delivers the auxiliary power to the load. The electric energy required by the load is also referred to as a “usage electric energy.” The auxiliary power applied to the load corresponds to the usage electric energy of the load exceeding the output electric energy of the stack.
However, in the conventional fuel cell system, because the electrically overloaded state of the fuel cell is remedied with the auxiliary power supply device applying the auxiliary power to the load, the auxiliary power supply device must produce a large amount of electrical power. This increases the operation cost of the entire fuel cell system.